I don't understand the concern about the clownfish dying. It is exceedingly rare that boxfish cause problems--the only ones I've experienced were when they killed themselves inside of a bag. My suggestion, unless you really want to remove this fish for the clams, is to let it stay. Otherwise, I don't think you have any big issues moving it, but it's not something you really need to do. Just be quick and gentle moving it if you decide to do it. You can always throw in a good portion of carbon if you get concerned.